Polymer flocculants with improved dewatering characteristics

ABSTRACT

A high molecular weight water soluble acrylamide based polymer having improved flocculating properties is provided. The polymer is prepared by a process, in which a small quantity of a chain branching agent is added to the polymerization reaction mixture continuously or semi-continuously while the monomers are polymerized. The polymer is useful in solid/liquid separations such as flocculants and coagulants in waste water treatment applications, and as thickening agents.

FIELD OF THE INVENTION

The present invention relates to materials and methods for the preparation of high molecular weight acrylic based water soluble polymers, and in particular to acrylamide based polymers for use as flocculants, dewatering and thickening agents.

BACKGROUND OF THE INVENTION

Synthetic polymeric materials such as polyacrylamides have long been used as flocculants in waste water treatment and other processes in which it is desirable to remove suspended solid materials. U.S. Pat. No. 3,235,490 discloses that certain crosslinked acrylamide based polymers are effective flocculants due to an electrostatic attraction followed by a sweeping action of a “filamentary” network provided by the crosslinked polymer. It is also disclosed that the formation of non-homogeneous products, which have a tendency to precipitate from solution, contributes to the effectiveness of the crosslinked polymers.

U.S. Pat. No. 4,720,346 describes a process for producing partially crosslinked acrylamide based cationic and anionic flocculants for use in municipal and industrial waste water treatment. The crosslinked polymeric material is said to have controlled non-linearity which is caused by crosslinking preformed polymers, or by polymerizing acrylamide monomers in the presence of a crosslinking agent. However, as disclosed therein, the intrinsic viscosity of the water soluble polymers decreases as the concentration of crosslinking agent is increased. Vigorous shearing action is required prior to or during flocculation to make the crosslinked or particulate polymeric material soluble and to increase the intrinsic viscosity. A high dosage of the polymeric material is required for effective removal of solids.

U.S. Pat. No. 4,759,856 describes a flocculation process for removing suspended solids in an aqueous suspension. A polymeric material comprising crosslinked water swellable polymer is added to water to form an aqueous composition, which is in turn added to an aqueous suspension for flocculation. The polymeric material must be subjected to shear during flocculation or in the aqueous composition before adding to the suspension.

Shearing or vigorous stirring of polymeric materials prior to or during flocculation requires process equipment which is often not available at the waste water treatment site. The additional shearing and stirring steps inevitably complicate the flocculent preparation, the flocculation process and reduce the cost efficiency.

SUMMARY OF THE INVENTION

This invention provides a high molecular weight water soluble polymer which exhibits enhanced flocculating or thickening performance in the absence of high shear. A process for preparing this polymer is also provided. In the process, a polymerization reaction mixture is provided which comprises one or more polymerizable acrylic monomers. The acrylic monomers in the reaction mixture are polymerized in the presence of a chain branching agent. Specifically, the chain branching agent is added to the reaction mixture continuously or semi-continuously, preferably until the polymerization reaction is substantially completed. The polymerization reaction can be conducted in inverse-emulsion, solution, solution-gel, or by a precipitation polymerization process.

Typically, the chain branching agent is a water soluble multifunctional monomer having at least two unsaturated groups, and is added at a total amount of from about 1 ppm to about 5×10⁴ ppm based on the total amount of the reaction mixture. In a preferred embodiment, N,N-methylenebisacrylamide, also referred to as MBA, is used as the chain branching agent at a total amount of from about 1 to about 1000 ppm, preferably from about 2 to about 300 ppm, more preferably from about 4 to about 100 ppm, and most preferably from about 4 to about 25 ppm based on total monomers. The methylenebisacrylamide is added continuously or semi-continuously during the polymerization reaction until a total monomer conversion of about 90% is achieved.

Suitable acrylic monomers for use in the process of this invention include, but are not limited to, acrylamide, methacrylamide, N-vinyl methyl acetamide, N-vinyl methyl formamide, dialkylaminoalkylmethacrylamide, sulphomethylated acrylamide, vinyl acetate, vinyl pyrrolidone, methacrylic esters, styrene, acrylonitrile, acrylic acid, methacrylic acid, itaconic acid, acrylamido methyl propane sulphonic acid, allylsulphonate, sodium vinyl sulphonate, sodium acrylate, diallyldimethylammonium chloride, methacrylamidopropyl trimethylammonium chloride, dialkylaminoalkyl methacrylate, dialkylaminoalkyl acrylate, dialkylaminoalkyl acrylate methyl chloride, and quaternaries or acid salts thereof. At least one of the above monomers is used. Two or more of the above monomers may be copolymerized.

In one aspect of the invention, the polymer of this invention is polyacrylamide, i.e., an acrylamide based high molecular weight water soluble polymeric material. At least one acrylamide is normally used as the monomer. For example, a preferred cationic monomer reaction mixture may contain acrylamide and dialkylaminoalkyl(meth)acrylate (as acid salt or quaternary salt) in a concentration of at least 30 wt % of the cationic monomer. A preferred anionic monomer system can include acrylamide and methacrylic acid as comonomers and acrylic acid as monomer.

In another aspect of this invention polyacrylamide is used as a flocculant for treating an aqueous suspension such as residential or industrial waste water. Accordingly, a process for flocculating an aqueous suspension of suspended solids is provided comprising adding to the suspension a flocculating amount of the polymer of this invention, such that an aqueous medium containing flocculated suspended solids is formed. The flocculated suspended solids are then separated from the aqueous medium in a conventional dewatering step, e.g., centrifuge, piston press, or belt press dewatering. Typically the flocculating amount, i.e., the effective dosage, of the polyacrylamide flocculant material of this invention ranges from about 1 to about 10⁵ ppm. The optimal dosage can vary, depending on the solid suspension to be flocculated and the properties of a particular polyacrylamide used. Preferably from about 10 to about 100 ppm of the polyacrylamide of this invention is used.

While not wishing to be bound by any theory, it is believed that in the polymerization process of this invention, regular branching and/or partial crosslinking along the polymer backbone is achieved as a result of the introduction of the chain branching agent into the reaction mixture in a constant manner or stepwise as a function of time. The polymeric material has increased homogeneity in its chemical structure and network and has sufficiently high intrinsic viscosity. As a result, the polymeric material, exhibits enhanced flocculating performance without being subjected to shearing action.

In addition, it is also believed that, as compared to randomly crosslinked or partially crosslinked polymeric flocculants known in the art, more attachment points are available in the polymer of this invention for attaching onto the solids in the aqueous suspension due to the regularly spaced branching and/or regular partial crosslinking. The polymeric material of this invention is a more facile and efficient flocculant. A lower dosage of the polymeric material of this invention is needed in a flocculating process while shear action or vigorous stirring either before or during the flocculating process is not required. Therefore, the high molecular weight polymer of this invention is superior in sludge dewatering and exhibits improved performance as thickening agents.

As compared to polyacrylamide materials prepared by conventional methods, the polyacrylarnide material according to this invention exhibits higher intrinsic viscosity, a lower level of aggregation/association, and a significantly higher weight-average sedimentation coefficient. Typically, the water soluble acrylamide based polymeric flocculent material according to this invention has an intrinsic viscosity of greater than about 4 dl/g (1M NaCl at 25° C.), and preferably greater than about 5 dl/g (1M NaCl at 25° C.), a sedimentation coefficient of the soluble portion of greater than about 5 Svedberg (0.05 M NaCl at 20° C.), preferably greater about 10 Svedberg (0.05 M NaCl at 20° C.), and an association/aggregation to chain branching agent ratio of less than about 3, preferably less than about 2.5.

Accordingly, this invention provides a process for preparing a water soluble acrylic based polymer which has regularly spaced branches and/or crosslinks along the polymer backbone. The polymer prepared according to this invention exhibits high intrinsic viscosity, high weight-average sedimentation coefficient, and low aggregation/association. The polyacrylamide of this invention is especially effective in flocculating suspension of solids without shearing.

The foregoing and other advantages and features of the invention, and the manner in which the same are accomplished, will become more readily apparent upon consideration of the following detailed description of the invention taken in conjunction with the accompanying examples, which illustrate preferred and exemplary embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a graph depicting the relationship between the amount of methylenebisacrylamide (“MBA”) added during polymerization, and the intrinsic viscosity (measured at 25° C. using 1 M NaCl as a solvent) of the resultant polyacrylamides as obtained in Examples 1 through 8;

FIG. 2(a) shows the synthetic boundary experiments performed in an analytical ultracentrifuge (Optima XL-I, Beckman at 20° C., 10,000 RPM, delay 10 minutes and 0.005 M NaCl) on samples obtained in Comparative Example 3. FIG. 2(a) does not show any meniscus movement. However, the concentration decreases as a result of a partial sedimentation of associates/aggregates. The separation reaches a limit at approximately 0.6 fringes;

FIG. 2(b) shows the synthetic boundary experiments performed in an analytical ultracentrifuge (Optima XL-I, Beckman at 20° C., 10,000 RPM, delay 10 minutes and 0.005 M NaCl) on samples obtained in Example 6. FIG. 2(b) does not show any meniscus movement. In FIG. 2(b), only a small change in concentration is visible implying a very minimal concentration decrease and a high soluble fraction;

FIG. 3 shows a graph of fringes (a measure of the detected polymer concentration) versus concentration obtained under synthetic boundary experiments using an analytical ultracentrifuge. Examples: +1, ◯2, 58 3, 5, ▪6, ▴8. FIG. 3 demonstrates that the polymer of Comparative Example 3 (8 ppm MBA, batch addition) exhibits a degree of aggregation higher than the polymer obtained in Example 8 (16 ppm MBA);

FIG. 4(a) shows the sedimentation profile obtained from a sample of the polymer in Comparative Example 3, in an analytical ultracentrifuge at 20° C., 30,000 RPM, delay 10 minutes and 0.05 M NaCl;

FIG. 4(b) demonstrates the sedimentation profile obtained from a sample of the polymer in Example 6, in an analytical ultracentrifuge at 20° C., 30,000 RPM, delay 10 minutes and 0.05 M NaCl. FIG. 4(b) shows a higher concentration signal (fringe displacement) than FIG. 4(a). This is related to differences in associated/aggregated regions. For the sample obtained in Comparative Example 3, the aggregation is 45-55%. However, the aggregation is less than 10% for the sample obtained in Example 6. In addition, the sedimentation front is less sharp for the sample obtained in Comparative Example 3. It broadens with time indicating a higher polydispersity/heterogeneity compared with the sample obtained in Example 6.

DETAILED DESCRIPTION OF THE INVENTION

The present invention will now be described more fully hereinafter with reference to the accompanying examples, in which preferred embodiments of the invention are shown. This invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art.

The present invention provides a process for the preparation of a regularly branched acrylic based high molecular weight water soluble polymer from one or more monomers. The polymer exhibits improved flocculating characteristics due to the introduction of regularly spaced branches and/or crosslinks as accomplished by the continuous or stepwise addition of small quantities of one or more chain branching agents.

The monomers used for the preparation of the polymeric material of this invention are monoethylenically unsaturated monomers. The monomers can be allyl monomers, but are generally vinyl, preferably acrylic.

Examples of non-ionic monomers include, but are not limited to, acrylamide, methacrylamide, N-vinylmethylacetamide, N-vinyl methyl formamide, vinyl acetate, vinyl pyrrolidone, methyl methacrylate, other acrylic or ethylenically unsaturated esters, styrene, and acrylonitrile.

Examples of cationic monomers are dialkylaminoalkylacrylates and methacrylates, especially dialkylamino ethyl acrylate, and their quaternary or acid salts, and dialkylaminoalkylacrylamides or methacrylamides and their quaternary or acid salts and Mannich products, such as quaternized dialkylaminoethylacrylamides. Alkyl groups are generally C₁₋₄.

Examples of anionic monomers includes, e.g., acrylic acid, sodium acrylate, methacrylate, itaconate, 2-acrylamide 2-methyl propane sulphonate, sulphopropylacrylate or methacrylate or other forms of these carboxylic or sulphonic acids.

One or more of the above monomers may be used in the process for preparing the polymer of this invention. In a preferred embodiment, at least one acrylamide is used as monomer to prepare a water soluble high molecular weight polyacrylamide having regularly spaced branches along the backbone of the polymer and a low polydispersity of branching. For example, a preferred cationic monomer reaction mixture may contain acrylamide and dialkylaminoalkyl(meth)acrylate (as acid salt or quaternary salt) with dialkylaminoalkyl(meth)acrylate being at least about 30 wt % of the total monomers. A preferred anionic monomer system can include acrylamide and methacrylic acid as comonomers and acrylic acid as monomer.

A chain branching agent must be incorporated into the process of this invention. As used herein, the term “chain branching agent” means a molecule, typically a polymerizable monomer, which when incorporated in the polymerization reaction for the preparation of the polymer of this invention, is capable of causing the formation of branching side chains along the backbone of the resultant polymer. Although crosslinks in the polymer may also be formed as a result of using the chain branching agent, excessive crosslinking should be avoided.

Typically, a chain branching agent is a water soluble multifunctional monomer having at least two unsaturated groups. Examples of chain branching agent include, but are not limited to, methylene-bis-acrylamide which is often referred to as MBA, divinylbenzene, diethylene glycol diacrylate, propylene glycol dimethacrylate, diallyl acrylate, diallylfumarate, trimethyloy propane triacrylate and the like. One or more chain branching agents can be used. Preferably, methylene-bis-acrylamide (MBA) is used as a chain branching agent in the process of this invention.

The one or more chain branching agents must be added to the polymerization reaction continuously or semi-continuously throughout the entire polymerization reaction process or in the period as defined below. The addition of the chain branching agent can begin before or shortly after the starting of the polymerization reaction, or begin simultaneously with the polymerization reaction. In any event, the addition of the chain branching agent must begin before about 50%, preferably about 25%, more preferably about 10%, most preferably about 5% of the monomer conversion is achieved. The addition of the chain branching agent can be stopped when the polymerization reaction process is substantially completed, i.e., when about 95%, more preferably about 92%, and most preferably about 90% of monomer conversion has been achieved.

The terms “continuously” and “continuous addition” are used herein to mean that the chain branching agent is added at all times throughout the polymerization reaction or during the period as defined above. The terms “semi-continuous”, “semi-batch”, and “stepwise as a function of the reaction time” are used herein interchangeably. They are used herein to mean that the chain branching agent is added to the polymerization reaction mixture intermittently throughout substantially the entire polymerization reaction process or the above-described duration.

Any manner of “continuous addition” or “semi-continuous addition” can be utilized, to produce preferably a regular pattern of branches and/or crosslinks along the backbone of the polymer of this invention. The regularity of the patterns can be embodied in the spacing of the branches and/or crosslinks along the backbone of the polymer, and/or in the length of the branches or crosslink bridges. As used herein, the term “regularity” means that the distances between any two adjacent branching points are same, or that such distances can change along the backbone of the polymer of this invention in a regular pattern, e.g., increasing or decreasing constantly or cyclically. In addition, the term “regularity” also refers to the lengths of the branches along the backbone of the polymer of this invention which can be all same or vary in a more regular pattern along the backbone relative to prior art.

As will be apparent to a skilled artisan, to produce such regularity, the rate of addition of chain branching agent, and/or the time interval between two consecutive additions in the case of “semi-continuous addition,” can be controlled in various manners. For example, the rate of addition can increase or decrease at a fixed accelerating or decelerating speed. Another example is that the rate of addition changes in a cyclical manner. In semi-continuous addition, the time intervals can change throughout the polymerization reaction according to a regular pattern.

Preferably, the addition of one or more chain branching agents is “in a constant manner.” As used herein, the term “in a constant manner” is intended to mean that in a continuous addition, one or more branching agents are added at the same rate substantially throughout the entire polymerization reaction or the duration as described above, and that in a semi-continuous addition, the addition of one or more chain branching agents is conducted in a manner such that both the time interval between two consecutive addition points and the amount added at each addition point are constant.

The manner of adding at once all of the crosslinking agent or chain branching agent required, typically before the starting of the polymerization reaction, is referred to herein as “batch addition.”

The concentration of the chain branching agent should be within the solubility limit of the chain branching agent in the aqueous or oil medium. When the polymerization reaction is conducted in an emulsion, typically it should be calculated in such a way that the aqueous or oil portion to be added in the course of the polymerization does not exceed 5 to 10 weight percent based on the total weight of emulsion.

The rate of addition of the chain branching agent can also vary with the degree of non-linearity desired or the desired final intrinsic viscosity, the total amount of the chain branching agent to be added, and the number of steps employed in the case of semi-continuous or stepwise addition. In addition, as will be apparent to a skilled artisan apprised of this disclosure, in order to form regularly spaced branches and/or crosslinks along the backbone of the polymer, excessive chain branching agent(s) should be avoided. Indeed, the amount of the chain branching agent(s) added in the polymerization should preferably be stoichiometrically limiting, i.e., significantly less than the available polymerization monomers at any give point of time during the polymerization reaction. Otherwise, regularity of branching may not be achieved and the polymers produced may be rendered insoluble and ineffective in flocculating or dewatering applications.

Typically, the total amount of chain branching agent added in the entire polymerization reaction process of this invention may be from about 1 to about 1000 ppm, preferably from about 4 to about 300 ppm, more preferably from about 4 to about 100 ppm, and most preferably from about 4 to about 25 ppm based on total monomers. Some minor degree of experimentation may be required to determine the total amount and the best manner of addition of a chain branching agent for a specific branched structure of a polymeric material of this invention, this being well within the capability of one skilled in the art once apprised of the present disclosure.

Additionally, the degree of branching can be controlled with the use of chain transfer agents in the polymerization recipe. Isopropanol can be used as a chain transfer agent in a preferred amount of from about 0.01 to 0.25 weight percent based on the total amount of reaction mixture or emulsion. Other chain transfer agents such as mercaptoethanol, propylene glycol, sodium hypophosphite, thioglycolic acid or lactic acid in preferred amounts of from about 0.01 to 2.0 weight percent based on the total amount of reaction mixture or emulsion can be employed.

Inverse emulsion polymerization, e.g., in a water-in-oil emulsion, is a preferred method for preparing the polymer of this invention although other processes, including solution polymerization, solution-gel polymerization and precipitation polymerization, can also be used for the preparation of the polymer of the present invention, as will be apparent to a skilled artisan.

In an inverse emulsion polymerization process, the resultant inverse polymer emulsion may contain a dispersed aqueous phase having the polymer of this invention, and a continuous oil phase formed of a water-immiscible inert organic liquid. In this type of inverse emulsions, the percent of the aqueous phase comprises from about 60 to about 95, preferably from 70 to 80, weight percent of the total emulsion weight. In addition to water, the aqueous phase also contains the monomers to be polymerized in preferred amounts of from about 40 to 60, weight percent based on the total weight of the emulsion.

The oil phase can be any inert aliphatic and/or aromatic hydrophobic liquid which does not interfere with the polymerization reaction. Examples of these hydrophobic liquids can be benzene, xylene, toluene, mineral oils, isoparaffinic oils, kerosenes, naphtas, and the like, and mixtures thereof. Preferably, the oil phase comprises about 15 to 30% of the emulsion.

The oil phase can also contain water-in-oil surfactants. Such surfactants are well known in the art and are used to promote the formation and stabilization of water-in-oil emulsions. Typically, the surfactants or stabilizers have HLB values in the range of from about 2 to about 10, preferably less than about 7. Examples of these surfactants or stabilizers include, but are not limited to, sorbitan esters of fatty acid, alkanolamides, fatty acid glycerides, glycerine esters, as well as ethoxylated versions of the above and any other well known emulsifers. The water-in-oil surfactants should be used in amounts not greater than about 5 weight percent of the total emulsion. Preferably the amount is not greater than 3%, and most preferably less than about 2.5%.

The polymerization reaction process of this invention can be carried out in the presence of a conventional polymerization initiator. Examples of suitable water-soluble initiators include, e.g., 2,2′-azobis-(2-amidinopropane) dihydrochloride, 4,4′-azobis-(4-cyanopentanoic acid), or redox system such as ammonium persulfate/ferric sulfate. Oil-soluble initiators include, e.g., dibenzoyl peroxide, dilauryl peroxide or tert-butyl peroxide, or azo compounds such as 2,2′-azobisisobutyronitrile, dimethyl 2,2′-azobisisobutyrate and 2,2′-azobis-(4-methoxy-2, 4-dimethylvaleronitrile). The initiators may be used either individually or in combinations and generally in an amount of about 0.015 to 0.5% by weight of the total emulsion.

The polymerization temperature can be selected based on the decomposition kinetics of the initiator used, and may be from about 5° C. to about 100° C. To reduce the residual monomer content, it is also possible to increase the temperature during the course of the polymerization. Alternatively, it is also possible to use additional initiators during and at the end of the polymerization.

The water-in-oil emulsions of the present invention can be self-inverting or can be inverted with the addition of a wetting agent. These wetting agents can be added to the water-in-oil emulsion or can be added to the water into which the emulsion is introduced. Preferably used wetting agents for inverting the water-in-oil emulsions are ethoxylated nonylphenol having a degree of ethoxylation between 5 to 20 or propoxylate fatty alcohols of 10 to 22 carbons, having a degree of alkoxylation between 5 to 20.

The acrylic based high molecular weight water soluble polymer of this invention typically has an intrinsic viscosity that is higher than that of similar polymers resulting from batch additions of a chain branching agent. It is believed that this is the result of the homogeneity, i.e., regularity in the polymer structure of the polymers of the present invention. Measurements by analytical ultracentrifuge confirm this hypothesis.

An analytical ultracentrifuge can be used to measure the level of non-linearity of the polymer of this invention. Analytical centrifugation has contributed significantly to the understanding of macromolecular systems since the pioneering work of Svedberg. See T. Svedberg and K. O. Pedersen, The Ultracentrifuge, Clarendon Press, Oxford (1940), which is incorporated herein by reference. Since then, analytical ultracentrifugation has been heavily used to characterize properties like average molar masses and molar mass distributions, sedimentation and diffusion coefficients and particles sizes. See Polymer News, 23:152 (1998), which is incorporated herein by reference.

Sedimentation velocity and the synthetic boundary experiments can be used in the analytical ultracentrifuge evaluation. In the first experiment, a high centrifugal field is applied to a solution of a polymer dissolved in slightly saline solvents. The polymer moves towards the bottom of the cell. The velocity of the species is dependent on its mass. Polymers with different molar masses are fractionated due to their sedimentation coefficients. In the second experiment, one sector of a special double sector cell is filled half with a polymer solution while the other half contains the solvent. The sectors are connected with a capillary channel. During the acceleration, the solvent layers the polymer solution. The height of the signal “jump” at the synthetic boundary is a measure of the polymer concentration. The decrease of the signal height detects loss of polymer by sedimentation. By using these experiments, truly dissolved polymer can be separated from the undissolved polymer in the form of associate/aggregate and precipitates.

The level of association/aggregation (measured in 0.05M NaCl) observed in the polymer of the present invention is typically below 10%, as compared to the typical 10-55% of the prior art polymers resulting from batch additions of chain branching agents. Additional measurements obtained in the analytical ultracentrifuge also indicate that the molar mass of the polymers of the present invention is higher than that of the polymers resulting from the batch addition of chain branching agent. For the same level of methylenebisacrylamide (MBA), the weight-average sedimentation coefficient at zero concentration for the soluble portion is generally above about 6.2 Svedberg, preferably above about 7 Svedberg, more preferably above about 10 Svedberg for the polymers of this invention, as compared to values of generally below about 6.2 Svedberg for the prior art polymers resulting from the batch addition of the chain branching agent. (1 Svedberg=10⁻¹³ second). These characteristics confer the polymers of the present invention superior advantages over the polymers resulting from the traditional batch addition of chain branching agent. The polymer prepared in accordance with the present invention exhibits excellent floc stability under shear, low polymer dosages both in belt filter presses and centrifuges, good water release or thickening ability, and low turbidity both in belt filter presses (filtrate) and centrifuges (centrate).

The polyacrylamide material of this invention is particularly useful for flocculating an aqueous suspension of suspended solids. The polymer can be used in wastewater treatment, e.g., municipal, farm, and industrial waste water. In addition, the polymer can also be used in industrial manufacturing, e.g., treating liquid suspensions to remove solids for purposes of e.g., modifying the color, taste, or viscosity of the aqueous suspension. Conventional flocculating methods known in the art can be used for this purpose. Typically, a flocculating process may include the steps of adding to the suspension a flocculating amount of a polyacrylamide flocculant material according to this invention to form an aqueous medium containing flocculated suspended solids. The flocculating amount, i.e., the effective dosage, of the polyacrylamide flocculant material of this invention may range from about 1 to about 10⁵ ppm, preferably from 4 to about 10⁴ ppm, more preferably from about 5 to about 1000 ppm, and most preferably from about 10 to about 500 ppm.

The polymers of the present invention are also useful as retention agent, in paper manufacturing. They can be added to the paper finish at the wet end of the paper machine in typical amounts such as about 0.05 to about 7 kilograms of active material per ton of finished paper.

As will be apparent to a skilled artisan, the effective dosage of the polymer for flocculating a liquid suspension can vary with the concentration of the solids contained in the liquid suspension to be treated, the chemical and physical structure of the polymer flocculant, and the like. Typically, more polymer is needed when the liquid suspension has a higher concentration of solids, and vice versa. When a polymer of this invention having more regularly spaced branches is used, less amount may be effective as compared to when a polymer having less regularly spaced branches is used. Some minor degree of experimentation may be required to determine the optimal dosage of a particular polymer of this invention for a particular liquid suspension to be treated, this being well within the capability of one skilled in the art once apprised of the present disclosure. In any event, the flocculating amount, i.e., the effective dosage typically falls within the ranges as described above.

Once an aqueous medium containing flocculated suspended solids is formed, it is dewatered by a conventional dewatering means, e.g., centrifuge, piston press, or belt press dewatering, or simply allowing the solids to precipitate.

The polymers of the present invention are particularly useful in a dewatering process involving belt filter presses and centrifuges. The performance in such equipment can be manifested by increased solids contents in the filter cake and a reduction in turbidity in the filtrate (belt filter presses) and centrate (centrifuges). It has, however, been found that there is a close correlation between flocculant performance in real applications and several laboratory tests such as capillary suction time (CST) and the conventional Buchner Funnel method. CST is measured as described in the Journal of Institute of Water Pollution Control, 67:233 (1968). This method involves the addition of a measured dose of flocculant solution to the sludge to be dewatered and applying a defined amount of shear for a measured time. Then, the conditioned sludge sample to be tested is placed in a stainless steel cylinder. The suction pressure of a filter paper beneath the sample cylinder draws out filtrate. The filtrate progresses advances radially in an elliptical pattern. The timer starts when the liquid reaches two electrodes and ceases when the liquid front reaches a third electrode. The best flocculants have the lowest CST values. The conventional Buchner Funnel is used to test the relative effectiveness of various flocculants to dewater a sludge. The test consists of obtaining a representative sludge sample, dividing it into aliquots, adding the desired amount of the polymer being tested, mixing the sludge with the polymer for 10 seconds at 400 rpm, pouring the conditioned sludge into the Buchner funnel with moist filter paper and then determining the drainage time to get a given volume or the filtrate volume after a given time. The best flocculants give the lowest times for a fixed volume or the highest volumes for a fixed time.

The following section summarizes exemplary preparation conditions and characteristics of the polymer flocculants of the present invention.

Comparative Example 1

This example illustrates the formation of a linear 60:40 mol % acrylamide:dimethylaminoeethylacrylate methyl chloride quaternary salt copolymer in water-in-oil emulsion. To a vessel equipped with agitation means were added 240 grams of Isopar M, 19.25 grams of sorbitan monooleate, and 5.75 grams of polyoxyethylene sorbitan trioleate as steric stabilizers. The mixture was then stirred rapidly at 300 RPM for 5 minutes. In a separate vessel also equipped with agitation means, the aqueous phase was prepared containing 144 grams of solid acrylamide, 320 grams of 80 wt % dimethylaminoethylacrylate methyl chloride quaternary salt, 0.25 gram of EDTA, 0.12 gram of potassium bromate, 2.0 grams of isopropanol, and 240.92 grams of demineralized water. The aqueous phase was buffered to a pH of 3.5 using concentrated sulfuric acid. The aqueous phase was stirred for 30 minutes at 250 RPM and then transferred slowly to the oil phase. Both phases were pre-emulsified at 300 RPM and then emulsified in a homogenizer at 8000 RPM for 30 seconds. The resulting emulsion was then added to a 1 liter stainless steel reactor equipped with agitation means, an inert gas sparge and a dissolved oxygen probe functioning in hydrophobic continuous phases. The emulsion was continuously sparged using 1 liter per minute of nitrogen. Simultaneously, the temperature of the reaction was gradually ramped up to 40° C. from its starting temperature of about 20° C. (ambient conditions). Once the dissolved oxygen concentration decreased to below 0.2 ppm, 0.11 gram of 2,2-Azobis(4-methoxy-2,4-dimethylvaleronitrile), an organically soluble free radical initiator, dissolved in 2.25 grams of xylene, was added through a septum in the top of the sealed reactor. The polymerization onset was noted by an increase in temperature (exotherm) of 0.2° C. or more under the influence of automatic temperature control. The reaction was maintained at 40° C. for 6 hours. At the end of the 6 hours, the polymerization temperature was increased to 55° C. and 0.23 gram of sodium metabisulfite, dissolved in 0.53 gram of demineralized water, were added to the reactor to reduce the residual acrylamide concentration to below 250 ppm. After cooling, 22.5 grams of ethoxylated nonylphenol were slowly added to the emulsion as an inverting surfactant. The resultant inverse-emulsion contained 40 wt % of active material. The linear polymer in aqueous solution had an intrinsic viscosity of 11.78 dL/g and a standard viscosity of 4.53 cps as measured in 1M NaCl at 25° C. using a Brookfield Programmable DV-II + with a UL adapter at 60 RPM. These characteristics and parameters from analytical ultracentrifugation experiments are summarized in Table 1.

Comparative Examples 2 Through 4

These examples illustrate the formation of prior art 60:40 mol % acrylamide: dimethylaminoethylacrylate methyl chloride quaternary salt copolymers at 40% active material in water-in-oil emulsion, in which MBA is added to the polymerization reaction at a batch manner. These cationic emulsion polymers were obtained using the conditions of Example 1 with the MBA totally added to the aqueous phase before the start of the reaction. The MBA was prepared in demineralized water at a concentration of 288 ppm. An aliquot of this solution was added to the aqueous phase to prepare water-in-oil emulsion of 4, 8 and 12 ppm of MBA, respectively. The intrinsic viscosity, standard viscosity and parameters from analytical ultracentrifugation experiments are also listed in Table I.

TABLE I Intrinsic viscosity (IV), standard viscosity (SV) and parameters from analytical ultracentrifugation experiments obtained for copolymers of 60:40 mol % acrylamide:DMAEA using MBA (batch addition) Sedimentation Coefficient Association/ MBA Association/ of the Aggregation (ppm Aggregation, Soluble to relative to IV, SV, in 0.05M Portion, MBA Example monomer) dL/g cps NaCl, % Svedberg ratio 1 0 11.78 4.53 0 — — 2 4 8.49 3.82 10-20 3.73 2.5-5 3 8 5.07 2.98 45-55 6.13 5.6-6.9 4 12 2.99 2.14 >60 — — 1 Svedberg = 10⁻¹³ s

Examples 5 Through 8

60:40 mol % acrylamide:dimethylaminoethylacrylate methyl chloride quaternary salt copolymers were formed at 40% active material in water-in-oil emulsion. These emulsion polymers were obtained under the same conditions as described in Comparative Example 1, except that a methylene-bis-acrylamide (MBA) solution was added continuously until a total monomer conversion of about 80-90%. At the end of the addition, the MBA concentration was 4, 8, 12 and 16 ppm (based on the total monomer weight), respectively. The MBA was prepared in demineralized water at a concentration of 288 ppm. The MBA solution was fed to the reactor with a constant-flow rate metering pump at flow rates between 1 to 3.2 mL/hour. The intrinsic viscosity, standard viscosity and parameters from analytical ultracentrifugation experiments are summarized in Table 2.

TABLE II Intrinsic viscosity (IV), standard viscosity (SV) and parameters from analytical ultracentrifugation experiments obtained for copolymers of this invention (60:40 mol % acrylamide:DMAEA) Sedimentation Coefficient Association/ MBA Association/ of the Aggregation (ppm Aggregation, Soluble to relative to IV, SV, in 0.05M Portion, MBA Example monomer) dL/g cps NaCl, % Svedberg ratio 5 4 9.62 4.47 0 6.17 0 6 8 10.38 4.53 <10 20.2 <1.25 7 12 10.10 3.96 <10 9.36 <0.83 8 16 5.61 3.01 30-40 14.3 1.9-2.5 1 Svedberg = 10⁻¹³ s

Example 9

To evaluate the effectiveness of the emulsion polymers of Examples 1-8, CST was measured on activated sludge of the waste water treatment plant of the city of Orbe, Switzerland for polymers obtained from each of the Examples 1 through 8. The results are shown in Table III. Since the CST of the polymers of the present invention were lower than those of the linear polymer prepared in Example 1 (without MBA), the polymers of the present invention were more effective than the linear polymer. Moreover, the branched polymers of Examples 5, 6, 7 and 8 according to this invention, were also more effective than those obtained by batch addition of MBA, with the polymer of Example 5 providing the best CST at lower doses.

TABLE III CST of polymers obtained in Examples 1 Through 8 Example Example Example Example 1 2 Example 3 Example 4 Example Example no chain 4 ppm 5 8 ppm 6 12 ppm 7 8 Dose ppm branching MBA, 4 ppm MBA, 8 ppm MBA, 12 ppm 16 ppm (Polymer) agent prior art MBA prior art MBA prior art MBA MBA 3.99 8.3 9.0 8.5 10.0 8.9 10.8 9.7 10.2 7.96 10.5 9.6 8.4 9.3 8.9 9.5 8.7 10.0 15.84 21.4 9.0 8.1 9.2 9.4 8.6 7.6 9.0 31.37 423.4 12.7 29.9 8.1 29.8 8.2 277.5 11.4 47.88 270.6 28.6 50.8 12.4 — 17.9 1034.1 — 63.84 — — 93.9 496.0 442.3 912.4 1179.1 — 79.80 — 158.5 — 151.8 — — — —

Example 10

This example was also carried out to evaluate the effectiveness of polymer of Example 5 as compared to an inverse emulsion copolymer, 55:45 mol % acrylamide:dimethylaminoethylacrylate methyl chloride quaternary salt copolymer at 48% active material. This latter inverse emulsion copolymer is currently used in the waster water treatment plant of the municipality of Orbe, Switzerland. Table IV shows the results of the evaluation of the Buchner Funnel test performed on an activated sludge sample. The polymer prepared in Example 5 clearly outperformed the polymer currently used in the waste water treatment plant. Better performance was obtained with the polymer of Examples 5 at lower doses.

TABLE IV Conventional Buchner Funnel test of the polymer obtained in Example 5 and a 55:45 acrylamide:dimethylaminoethylacrylate methyl chloride quaternary salt copolymer at 48% active material using an activated sludge sample of the waste water treatment plant of the municipality of Orbe, Switzerland Dose, ppm Polymer currently Dose, used in the ppm waste water Polymer Time to 40 mL, treatment plant Time to 40 mL, Example 5 sec of Orbe sec 4 9 4.8 10 8 9 9.6 10 12 9 14.4 10 16 97 19.2 9

Example 11

The emulsion polymer of Example 5 was also compared with the cationic powder flocculants Superfloc C-448 and Superfloc C-448 A obtained from Cytec Industries. Table V shows the Buchner Funnel test results of the comparison. An activated sludge sample with 3.54% solids of a waste water treatment plant of the city of Johannesburg, South Africa was employed in the evaluation. Again, the emulsion polymer of Example 5 showed excellent performance compared with the cationic powders based on the same active material. This would represent considerably economic advantages in using the polymer of Example 5 since the emulsion polymer of Example 5 had 40% active material compared with 95-100% active material of the prior art powder material.

TABLE V Conventional Buchner Funnel test of the polymer obtained in Example 5 and Superfloc 448-A and 448 (Cytec Industries) using an activated sludge sample of a waste water treatment plant of the city of Johannesburg, South Africa Dose, ppm Time Dose, ppm Time Dose, ppm Time Polymer to 40 Superfloc to 40 Superfloc to 40 Example 5 mL, sec 448-A mL, sec 448 mL, sec 76 Very high 95 78 95 19 112 12 140 11 140 9 145 60 182 21 182 11 178 77 222 23 260 25 209 80 260 22 333 53

Example 12

This example illustrates the scale-up of the preparation of a branched copolymer of acrylamide and dimethylaminoethylacrylate in water-in-oil emulsion with a acrylamide:dimethylaminoethylacrylate methyl chloride quaternary salt molar ratio of 60:40. To a 40 liter stainless steel vessel equipped with agitation were added 5760 grams of Isopar M, 416 grams of sorbitan monooleate and 124 grams of polyoxyethylene sorbitan trioleate as steric stabilizers. The mixture was then stirred rapidly at 300 RPM for 5 minutes. In a 25 liter stainless steel reactor, the aqueous phase was prepared containing 3456 grams of solid acrylamide, 7680 grams of 80 wt % dimethylaminoethylacrylate methyl chloride quaternary salt, 2 grams of EDTA, 4 grams of potassium bromate, 32 grams of isopropanol and 5383 grams of demineralized water. The aqueous phase was buffered to a pH of 3.5 using concentrated sulfuric acid. The aqueous phase was stirred for 30 minutes at 250 RPM to dissolve the solid materials, and then transferred slowly to the oil phase. Both phases were pre-emulsified at 300 RPM for 5 minutes and then emulsified in-a homogeneizer at 3000 RPM for 5 minutes. The resulting emulsion was then transferred back to the 25 liter stainless steel reactor equipped with an inert gas sparge and a dissolved oxygen probe. The emulsion was continuously sparged using 3-5 liter per minute of nitrogen. Simultaneously, the temperature of the reaction was gradually ramped up to 40° C. from its starting temperature of about 20° C. (ambient conditions). Once the dissolved oxygen concentration was below 0.2 ppm, 1.2 grams of 2,2-Azobis(4-methoxy-2,4-dimethylvaleronitrile), dissolved in 3.6 grams of xylene, were added through a septum in the top of the sealed reactor. The polymerization outset was noted by an increase in temperature (exotherm) of 0.2° C. or more under the influence of automatic temperature control. The reaction was maintained at 40° C. for 6 hours. Immediately after the start of the polymerization, MBA solution in demineralized water (200 ppm) was added to the reactor with a constant-flow rate metering pump at a flow rate of 42 mL/hour. The addition of MBA continued until the total monomer conversion was 90%. Therefore, at the end of the polymerization, MBA was added to the reactor at a concentration of 4.37 ppm based on the total weight of monomers. During the course of the polymerization, additional initiator was injected as necessary to maintain a 20% polymerization rate per hour. At the end of the 6 hours, the polymerization temperature was increased to 55° C. and 21.6 grams of sodium metabisulfite, dissolved in 50.42 grams of demineralized water, were added to the reactor to reduce the residual acrylamide concentration to below 250 ppm. After cooling, 720 grams of ethoxylated nonylphenol were slowly added to the emulsion as an inverting surfactant. The resultant inverse-emulsion contained 40 wt % of active material. The polymer in aqueous solution had a standard viscosity of 4.66 cps as measured in 1M NaCl at 25° C. using a Brookfield Programmable DV-II+ with a UL adapter at 60 RPM.

Example 13

The 60:40 mol % acrylamide:dimethylaminoethylacrylate methylchloride quaternary salt polymer (40% active material) prepared in Example 12 was compared against 55:45 mol % acrylamide:dimethylaminoethylacrylate methyl chloride quaternary salt polymer (48% active material) in a belt filter press of the waste water treatment plant of the municipality of Orbe, Switzerland. The evaluation was conducted with activated sludge over a two week period. In the first week, the 55:45 mol % acrylamide:dimethylaminoethylacrylate methyl chloride quaternary salt polymer was evaluated while the polymer of Example 12 was evaluated during the second week. Samples were taken to measure weight % dry material in the activated sludge, chemical oxygen demand in the filtrate, weight % dry material in the cake solids and the size of the flocs generated during the evaluation. Table VI shows the results of the evaluation. Reductions up to 28% in polymer doses and larger and more stable flocs were observed with the polymer of Example 12 compared with the 55:45 mol % acrylamide:dimethylaminoethylacrylate methyl chloride quaternary salt polymer (48% active material). Other variables such as thickening and COD were comparable.

TABLE VI Evaluation results of a 55:45 mol % acrylamide:dimethylaminoethylacrylate methyl chloride quaternary salt polymer (48% active material) and the polymer obtained in Example 12 using a belt filter press and activated sludge of the waste water treatment plant of the municipality of Orbe, Switzerland COD, Floc size, Dose, ppm % Polymer Thickening g/L mm Day 55:45 60:40 55:45 60:40 55:45 60:40 55:45 60:40 55:45 60:40 1 35.6 0.55 6.4 43.5 4 2 35.6 0.63 7.75 68 4 3 42.5 0.61 7.3 76 4-5 4 42.5 0.63 7.95 67.5 4 5 42.5 0.62 6.5 72 4-5 6 28.4 0.45 6.25 81 7-8 7 28.4 0.47 6.9 62 7-8 8 24.2 0.63 7.95 67.5 7-8 9 31.2 0.49 6.5 30 7-8 10 28.4 0.5 7.1 45 7-8

Notes:

1) 55:45 refers to a 55:45 mol % acrylamide: dimethylaminoethyl acrylate methyl chloride quaternary salt polymer (48% active material).

2) 60:40 refers to the polymer of Example 12.

3) % Polymer is the polymer wt % based on the dry solids of the activated sludge.

4) Thickening is the ratio between the cake solids and the solids in the fresh activated sludge.

5) COD is the chemical oxygen demand of the filtrate.

6) % Polymer represents the percentage of active material over the total amount of fresh sludge.

7) Thickening is defined as the ratio of total solids of the final sludge over the total solids of the fresh sludge.

Many modifications and other embodiments of the invention will come to mind to one skilled in the art to which this invention pertains having the benefit of the teachings presented in the foregoing descriptions and the associated drawings. Therefore, it is to be understood that the invention is not to be limited to the specific embodiments disclosed and that modifications and other embodiments are intended to be included within the scope of the appended claims. Although specific terms are employed herein, they are used in a generic and descriptive sense only and not for purposes of limitation. 

That which is claimed:
 1. A process for preparing a water soluble acrylic based polymer, comprising the steps of: providing a polymerization reaction mixture comprising one or more polymerizable acrylic monomers; and polymerizing said acrylic monomers while adding to the reaction mixture a chain branching agent continuously or semi-continuously.
 2. The process of claim 1, wherein said one or more acrylic monomers are selected from the group consisting of acrylamide, methacrylamide, N-vinyl methyl acetamide, N-vinyl methyl formamide, dialkylaminoalkylmethacrylamide, sulphomethylated acrylamide, vinyl acetate, vinyl pyrrolidone, methacrylic esters, styrene, acrylonitrile, methacrylic acid, itaconic acid, acrylamido methyl propane sulphonic acid, allylsulphonate, sodium vinyl sulphonate, sodium acrylate, diallyldimethylammonium chloride, methacrylamidepropyl trimethylammonium chloride, dialkylaminoalkyl methacrylate, dialkylaminoalkyl acrylate, dialkylaminoalkyl acrylate methyl chloride, and quaternaries or acid salts thereof.
 3. The process of claim 1, wherein said chain branching agent is N, N-methylenebisacrylamide.
 4. The process of claim 1, wherein the chain branching agent is added to the reaction mixture continuously in a constant manner.
 5. The process of claim 1, wherein the chain branching agent is added to the reaction mixture semi-continuously in a constant manner.
 6. A process for preparing a water soluble acrylamide based polymer, comprising the steps of: providing a polymerization reaction mixture comprising one or more polymerizable acrylamide monomers; and polymerizing said acrylamide monomers while adding to the reaction mixture a chain branching agent continuously or semi-continuously.
 7. The process of claim 6, wherein said chain branching agent is added at a total amount of from about 1 ppm to about 5×10⁴ ppm based on said reaction mixture.
 8. The process of claim 6, wherein said acrylamide monomers are selected from the group consisting of acrylamide, methacrylamide, N-vinyl methyl acetamide, N-vinyl methyl formamide, dialkylaminoalkylmethacrylamide, sulphomethylated acrylamide.
 9. The process of claim 6, wherein said reaction mixture further comprises a monomer selected from the group consisting of vinyl acetate, vinyl pyrrolidone, methacrylic esters, styrene, acrylonitrile, acrylic acid, methacrylic acid, itaconic acid, acrylamido methyl propane sulphonic acid, allylsulphonate, sodium vinyl sulphonate, sodium acrylate, diallyldimethylammonium chloride, methacrylamidepropyl trimethylammonium chloride, dialkylaminoalkyl methacrylate, dialkylaminoalkyl acrylate, dialkylaminoalkyl acrylate methyl chloride, and quaternaries or acid salts thereof.
 10. The process of claim 6, wherein N, N-methylenebisacrylamide is added as the chain branching agent at a total amount of from about 1 ppm to about 100 ppm based on monomers.
 11. The process of claim 6, wherein said reaction mixture is in a form of inverse-emulsion.
 12. The process of claim 6, wherein said one or more monomers include acrylamide and dimethylaminoethylacrylate (as acid salt or quaternary salt).
 13. The process of claim 6, wherein said chain branching agent is added continuously in a constant manner.
 14. The process of claim 6, wherein said chain branching agent is added semi-continuously in a constant manner.
 15. A process for preparing a water soluble acrylamide based polymer, comprising the steps of: providing a polymerization reaction mixture in an inverse-emulsion comprising acrylamide and dimethylaminoethylacrylate (as acid salt or quaternary salt) as polymerizable monomers, with at least 30% by weight of the monomers being said dimethylaminoethylacrylate (as acid salt or quaternary salt); and polymerizing said monomers while adding to the reaction mixture N, N-methylenebisacrylamide as a chain branching agent, in a constant manner, at a total amount of from about 1 ppm to about 100 ppm based on total monomers.
 16. A process for preparing a water soluble acrylic based polymer, comprising the steps of: (a) providing a polymerization reaction mixture comprising one or more polymerizable acrylic monomers; and (b) polymerizing said acrylic monomers while adding to the reaction mixture a chain branching agent either continuously or stepwise as a function of reaction time, wherein said water soluble acrylic based polymer exhibits an intrinsic viscosity that is at least about 10% higher than the intrinsic viscosity achieved by performing the polymerization step (b) with batch addition of chain branching agent instead of continuous or stepwise addition.
 17. A process for preparing a water soluble acrylic based polymer, comprising the steps of: providing a polymerization reaction mixture comprising one or more polymerizable acrylic monomers; and polymerizing said acrylic monomers while adding to the reaction mixture a chain branching agent either continuously or stepwise as a function of reaction time, wherein said water soluble acrylic based polymer exhibits a an intrinsic viscosity of greater than about 4 dl/g in 1M NaCl at 25° C. and a sedimentation coefficient of the soluble portion of greater than or equal to 6.17 Svedberg in 0.05 M NaCl at 20° C.
 18. A process for preparing a water soluble acrylic based polymer, comprising the steps of: providing a polymerization reaction mixture comprising one or more polymerizable acrylic monomers; and polymerizing said acrylic monomers while adding to the reaction mixture a chain branching agent either continuously or stepwise as a function of reaction time, wherein said water soluble acrylic based polymer exhibits an intrinsic viscosity of greater than about 4 dl/g in 1M NaCl at 25° C. and an association to chain branching agent ratio of less than 2.5 as measured in 0.05 M NaCl at 20° C.
 19. A process for preparing a water soluble acrylic based polymer, comprising the steps of: providing a polymerization reaction mixture comprising one or more polymerizable acrylic monomers; and polymerizing said acrylic monomers while adding to the reaction mixture a chain branching agent either continuously or stepwise as a function of reaction time, wherein said adding further comprises adding said chain branching agent at a constant rate of between about 0.1 to about 20 weight % of the total amount of chain branching agent per hour during polymerization. 